For example, an electrophotographic copying machine is equipped with a heat-fusing member (roll or belt) to hot-press with a heating roller a toner image transferred from sensitizing drum to paper and like subject to be transferred. The heat-fusing member requires, for example, the following functions:
sufficient resistance to temperature at which toner image is melted and fixed on a subject; PA1 good release properties from toner image leading to free of toner on fusing surface; PA1 rapid elevation of fusing surface to a required temperature; PA1 free of expansion and contraction and strength reduction when heated at high temperature for a long period of time; and PA1 good chemical resistance.
Japanese unexamined patent publication H5-212837 (hereinafter referred to as "reference A") Japanese unexamined patent publication H7-178741 (hereinafter referred to as "reference B") and Japanese unexamined patent publication H7-246671 (hereinafter referred to as "reference C") disclose a fusing member with necessary functions as shown above.
Reference A relates to a composite tube prepared by fitting a tubular fluororesin film (hereinafter tube B) on a tubular thermoplastic polyimide film (hereinafter tube A), followed by welding them at, for example, 280-400.degree. C. It is found that the composite tube of reference A is insufficient in the following points.
For example, welding tube A on the outside of which tube B is fitted may cause a little deformation to make the composite tube out of round. When the composite tube is used as fusing member, each heating on fusing causes only a little dimensional change (contraction) which leads to large dimensional change in long-term use. In addition, entire strength (Young's modulus) is gradually decreased. As a result, the composite film is estimated as insufficient durability.
In contrast, references B and C disclose a composite tubular material wherein a tubular fluororesin layer is formed on the surface of polyimide resin tube directly or indirectly (primary coat is formed separately). The fluororesin coating layer is formed by dipping the resin in a dispersion, spraying, roll-coating and like coating means. The polyimide/fluororesin composite tube prepared by such coating means has the disadvantages of poor surface smoothness of fluororesin layer, pinhole which is likely to be generated, and ununiformity of film thickness. The disadvantages result in incapability of heat-fusing toner image on a subject to be transferred uniformly with superior image quality leading to dirty non-image portion by toner.
According to said coating means, there is a problem that weak adhesion on the surface of polyimide surface leads to peeling of fluororesin layer, in particular, in long-term use. Peeling of fluororesin layer also leads to removal of toner on the surface of subject during heat-fusing.